1. Field of the Invention
The present invention relates to devices for separating the lighter fraction from the heavier fraction of a liquid sample. More particularly, this invention relates to self-contained devices for separation of the liquid component from the cellular component of a blood sample without the use of a centrifuge, an auxiliary circulating system, an auxiliary pump or the like.
2. Description of the Prior Art
A common method of obtaining a blood sample involves the use of a two-cannula needle assembly and an evacuated glass tube having a pierceable stopper. The method involves inserting one cannula of the needle assembly into the subject's vein and piercing the stopper of the evacuated glass tube with the other cannula of the needle assembly, thereby establishing fluid communication between the vein and the interior of the glass tube. Due to the lower pressures within the evacuated glass tube blood is drawn from the subject into the tube. Evacuated tubes and needle assemblies for use therewith are described in U.S. Pat. No. 3,469,572 to Nehring. Such devices are commercially available under the VACUTAINER Brand name from Becton, Dickinson and Company, Paramus, N.J.
The blood sample, in the evacuated glass tube, may then be placed in a centrifuge and centrifuged until the more dense cellular component of the blood sample is driven to the bottom of the tube and the less dense plasma is positioned at the top of the sample. The stopper from the tube may then be removed and the plasma sample poured off for subsequent testing. If serum is desired the blood sample is allowed to clot before centrifuging the blood sample.
U.S. Pat. No. 4,057,499 to Buono teaches the collection of a plasma or serum sample using a sampling member having a hollow interior for the collection of liquid and a piston connected to one end of the sampling member. The piston includes a lip for forming a seal with the interior walls of the sample containing glass tube and an interior portion contains a filter and a one-way valve. In use, a blood sample, in a glass tube, is centrifuged to separate the liquid and the cellular phases and then the device of Buono is placed in the tube and forced along the inner tube surface so that the piston passes through the liquid portion of the sample forcing the liquid portion through the filter and through the valve into the hollow interior of the device. The device containing a portion of the liquid sample is then removed from the glass tube. Buono teaches that it is desirable to physically separate the liquid phase of the sample from the cellular phase to prevent deleterious chemical interaction between the two. Accordingly, Buono teaches the use of a centrifuge and a separate filtering apparatus to obtain the plasma or serum sample.
It is believed that forcing a filter membrane through a blood sample, without the use of a centrifuge, is not a practical method of separating the liquid and the cellular components because the filter membrane would soon become caked with the cellular components and unable to perform its task as a filter. Accordingly, it is believed that if the liquid phase of the blood sample is to be filtered from the cellular portion of the blood sample, without the use of a centrifuge, a cross-flow filter arrangement should be provided. In a cross-flow arrangement, the blood sample flows across the surface of the filter membrane in a direction parallel to the major axis of the membrane while a secondary force provides a pressure differential between the blood side of the membrane and the liquid side so that the liquid phase will pass through the membrane. U.S. Pat. Nos. 3,211,645; 4,191,182; 4,212,742 and 4,343,705 teach various devices for the filtration of liquid using a cross-flow technique. In each of these patents, the devices utilized require an applied pressure driving force from a separate source across the membrane filter in order to bring about the proper separation. That is, a separate pumping and/or circulating device must be used in conjunction with the cross-flow filter.
It is believed that the cross-flow method of filtration is superior because when the blood sample is caused to flow in parallel relationship across the filter membrane there is substantially less tendency for the membrane to be clogged by the cellular portion of the blood sample and therefore allowing the blood to be filtered without the use of a centrifuge. In any event, all of the devices and methods described above require the use of relatively expensive support devices such as a centrifuge, liquid pumping and/or circulating systems, or the like. In addition, dependency on additional equipment can consume valuable time in emergency situations wherein the liquid phase of the patient's blood sample must be analyzed properly in order to diagnose the problem and/or to provide proper emergency treatment. In these situations, it is desirable to provide a device for the separation of the liquid phase of the blood sample from the cellular phase immediately upon the drawing of the blood sample without further steps.
The prior art teaches various devices and methods to separate the liquid phase from the cellular phase of a blood sample. All of the above-mentioned prior art devices require the use of a centrifuge or auxiliary pumping or circulating systems to provide the liquid phase sample. There is still a need for a simple, straightforward, reliable, easily fabricated device for the separation of the lighter fraction from the heavier fraction of a liquid sample which will operate without the use of additional equipment such as centrifuges and pumps, to promptly produce an isolated quantity of the lighter fraction of the liquid sample.